High power semiconductor diode laser devices emitting at a wavelength (.lambda..sub.g) in a range between about 0.80 micrometers (.mu.m) and 0.87 .mu.m are of interest in certain applications, such as pump sources for Nd:YAG lasers. Conventionally, for .lambda..sub.g .ltoreq.0.85 .mu.m, such lasers use aluminum gallium arsenide (AlGaAs) as an active layer. Such lasers, however, are known to suffer from short lifetimes and limited output power compared with the aluminum-free compound gallium arsenide( GaAs).
Quaternary alloys in the system indium gallium arsenide phosphide (InGaAsP), lattice matched to GaAs, offer an attractive alternative to the AlGaAs/GaAs system. They offer a potential of greater reliability, due to a resistance to formation of so-called dark-line defects, and due to a larger mirror-facet damage limit, since InGaAsP has significantly lower surface recombination velocity than AlGaAs. They also offer a potential for growing reliable diode lasers on silicon substrates rather than on gallium arsenide substrates, which are conventionally used. Properties and methods of growth of such aluminum-free lasers are discussed in detail in U.S. Pat. No. 5,389,396 to Razeghi, and in a paper "High-Power 0.8 .mu.m InGaAsP-GaAs SCHSQW Lasers", Garbuzov, D. Z., et al., IEEE Journal of Quantum Electronics, Vol 27, No. 6, June 1991, pp. 1531-1535.
A particular problem with lasers formed from the InGaAsP/GaAs material system is that only small conduction band discontinuities are possible between adjacent layers. This causes the laser to suffer massive carrier leakage from the active region. Because of this, such lasers have a relatively high threshold-current density for lasing, low internal efficiency (.eta..sub.i), and a low threshold-current characteristic temperature (T.sub.0). A low T.sub.0 corresponds to a more rapid increase in threshold-current with increasing heatsink temperature.
One attempt to solve the problem of carrier leakage in lasers having an active region formed from the InGaAsP/GaAs material system has been made by forming cladding layers of an aluminum-containing material (AlGaAs) and forming only the active layer and bounding waveguide or confinement layers from compounds in the InGaAsP/GaAs material system. This work is described in detail in a paper "Highly Reliable Operation of High-Power InGaAsP/InGaP/AlGaAs 0.8 .mu.m Separate Confinement Heterostructure Lasers", Fukunaga, T. et al., Japan Journal of Applied Physics, Vol. 34, Part 2, No 9B, pp. L1175-L1177, (September 1995). Positive results disclosed in this paper were that inclusion of aluminum containing cladding layers does not appear to adversely affect reliability of the lasers, and that a higher T.sub.0 was obtained than had been obtained for aluminum free lasers. Unfortunately, the lasers appeared had a relatively high J.sub.th.
There is a need for semiconductor diode lasers which have the advantages of lasers having an aluminum-free active layer, but which have high internal efficiency, a high T.sub.0, and a low J.sub.th. Notwithstanding any improvements already made in increasing efficiency and T.sub.0, further improvements therein are needed to improve reliability of such lasers and to expand their range of applications.